When braking in a straight line, factors that are a function of longitudinal vehicle dynamics are considered when trying to attain optimum vehicle braking effectiveness while maintaining optimum vehicle control. At any given position, there is a total amount of friction between the wheels and the surface of a road. When braking in a straight line, essentially all that friction is available for stopping the vehicle. When braking while turning, however, some of that friction is required to keep the wheels from sliding sideways. This condition results in a jackknife if the tractor drive wheels slip too much or trailer sway if the trailer wheels slip too much. Whatever friction is used to counteract lateral wheel slip is, of course, not available to counteract longitudinal wheel slip.
Antilock braking systems have used factors that are a function of longitudinal vehicle dynamics in trying to attain optimum vehicle braking effectiveness while maintaining optimum vehicle control. Such systems determine at least vehicle and wheel speeds and control the pressure applied to vehicle brakes to restrict wheel slippage to an effective level.
Commonly, antilock braking systems attempt to maintain an average, preset wheel slip during braking. Such system designs dictate performance compromises between providing minimum-distance braking on straight paths and optimizing vehicle control during braking on curved paths, especially at relatively high velocities.
Somewhat more sophisticated antilock braking systems have included, in the factors used to control the application of brake pressure most effectively, factors that are a function of lateral as well as longitudinal vehicle dynamics. Such factors are applicable in situations where the vehicle is braked while on a curve or while otherwise turning.
U.S. Pat. No. 4,758,053, issued to Yasuno et al., discloses an automotive vehicle antiskid brake control system that includes a hydraulic brake circuit that increases and distributes brake fluid pressure to wheel cylinders in response to a manual braking operation. A pressure control valve unit is also included and allows wheel cylinder brake fluid pressure either to increase in response to the manual braking operation or to decrease.
Sensors generate angular wheel velocity and lateral force signals that are used by a controller to derive a control signal that operates the pressure control valve unit. The controller derives a wheel slippage value based on the angular wheel velocity signal and directs the pressure control valve unit to decrease brake fluid pressure to the wheel cylinders when the wheel slippage value exceeds a reference wheel slippage value. The controller derives the reference wheel slippage value based on the lateral force signal and decreases the reference wheel slippage value when the lateral force signal exceeds a predetermined value.
U.S. Pat. No. 5,188,434, issued to Ruf et al., discloses a vehicle brake pressure controller that regulates brake fluid pressure in each wheel cylinder according to a comparison of instantaneous slip values to variable desired slip values. The desired slip values are varied as a function of the angle of inclination of one wheel of each axle.
While each of these braking systems functions with a certain degree of efficiency, none disclose the advantages of the improved method and apparatus for increasing control of a braking vehicle of the present invention as is hereinafter more fully described.